I believe, after reading the DuckDNS “support” group, that the service has been having intermittent issues over the last several weeks/months. There is no further information. I would suggest you move to a free DDNS service, from a provider that–as @arrogantrabbit pointed out–has other paid services that support this “free” service. I moved over to DYNU.com.
I used to actually use no-ip.com a while ago and guess I should have stuck with it.